Simplify the following expression: $k = \dfrac{4rs}{12rs - 12s} + \dfrac{2ts - 6rs}{12rs - 12s}$ You can assume $r,s,t \neq 0$.
Since the expressions have the same denominator we simply combine the numerators: $k = \dfrac{4rs + 2ts - 6rs}{12rs - 12s}$ $k = \dfrac{-2rs + 2ts}{12rs - 12s}$ The numerator and denominator have a common factor of $2s$, so we can simplify $k = \dfrac{-r + t}{6r - 6}$